The present invention relates to a method for adjusting the average particle size of a spherical catalyst support, in particular a spherical catalyst support comprising a magnesium dihalide-ethanol-adduct, a spherical catalyst support obtained by the method, a solid catalyst composition comprising the spherical catalyst support, and the use of the solid catalyst composition for the polymerization of an olefins.
It is known that spherical solid catalyst compositions for the polymerization of α-olefins are favorable, since the obtained olefin polymer usually reproduces the shape of the catalyst component. A spherical shaped olefin polymer powder has a high bulk density and is easy to handle in polymerization plants. In particular, the pneumatic conveying of the olefin polymer powder is positively affected by the spherical shape. Over the last four decades the improvement of the preparation method for spherical catalyst supports comprising a magnesium compound and an alcohol being used to prepare said spherical solid catalyst compositions was subject of many publications.
For example, U.S. Pat. No. 4,399,054 describes a method for preparing spherical catalyst supports for propylene polymerization catalysts with defined physical properties by forming an emulsion of a magnesium dihalide-alcohol adduct with an inert liquid at an elevated temperature and quenching the emulsion to cause rapid solidification of the adduct in form of spherical particles. The inert liquid should be immiscible with the adduct and inert towards reaction with the adduct. The temperature should be high enough to melt the adduct in the inert liquid. The examples disclose vaseline oil either alone or as a mixture with a silicone oil as the preferred inert liquid for the preparation of the supports.
Another method for the preparation of spherical catalyst support particles is described in DE 2924029. The method consists of the following steps: a) Forming a homogenous mixture comprising a MgCl2-alcohol adduct and an inert liquid, b) pumping the emulsion through a tube with defined dimensions under turbulent flow and c) cooling the emulsion rapidly to solidify the adduct by immersing it into a cooled liquid. DE 2924029 also discloses the linear velocity of the emulsion in the tube and the surface tension between the adduct and the inert liquid as critical parameters which determine the size of the spherical particles. The inert liquids used for the formation of the emulsion are kerosene and vaseline oil. Overall, the method is similar to the method described in U.S. Pat. No. 4,399,054.
EP 0 018 737 also discloses a method for the preparation of spherical catalyst supports. The method includes the addition of at least one auxiliary component selected from the group of surface-active agents, which are not siloxanes, and the group of siloxanes to the molten adduct suspended in an inert hydrocarbon liquid. After vigorous stirring for a certain period, the mixture is quenched to solidify the spherical particles of the adduct.
A spray drying method for the preparation of large spherical catalyst support particles of an adduct is disclosed in EP 0 700 936 B1. This method includes a dealcoholation step at reduced pressure.
Yet another method is disclosed in U.S. Pat. No. 6,962,889, which includes the use of alkyl silicates to form spherical particles of an adduct with relatively large size. The alkyl silicate, for example tetraethyl orthosilicate, is added to a solution of MgCl2 in an alcohol, usually ethanol. After proper mixing, a certain amount of an inert hydrocarbon liquid can be added to the above described solution to get an emulsion of the adduct. Finally, the solution or emulsion is transferred into cooled hexane to solidify the adduct and yield spherical particles having a relatively large particle size.
U.S. Pat. No. 7,135,531 describes a method for the preparation of spherical catalyst supports. The method comprises the use of an ether, a surfactant and an alkyl silicate to form spherical MgCl2-alcohol adducts. The combination of an ether, a surfactant and an alkyl silicate positively influences the formation of spherical particles having a controllable average particle size.
All prior art methods for the production of spherical catalyst supports comprising an MgCl2-alcohol adduct are more or less complicated and cannot be used to prepare spherical catalysts with controllable small average particle sizes and at the same time preferably also a comparatively narrow particle size distribution.